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Structural and Optical Properties of Heat Treated Zn2SiO4 Composite

Prepared by Impregnation of ZnO on SiO2 Amorphous Nanoparticles

E. A. G. Engku Ali1,2,3∗, K. A. Matori3, E. Saion4, S. H. A. Aziz4, M. H. M. Zaid4and I. M. Alibe4

1School of Fundamental Science, Universiti Malaysia Terengganu,

21030 Kuala Nerus, Terengganu, Malaysia.

2Advanced Nano Materials (ANoMa) Research Group,

Universiti Malaysia Terengganu, 21030 Kuala Nerus, Terengganu, Malaysia.

3Materials Synthesis and Characterization Laboratory, Institute of Advanced Technology,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor

4Department of Physics, Faculty of Science,

Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia.

In this study, Zn2SiO4 composite-based ceramic was synthesised using amorphous SiO2

nanoparticles as a silicon source. Different ratios of Zn:Si were prepared by mixing amor-phous SiO2 nanoparticles with aqueous zinc nitrate. Amorphous SiO2 nanoparticles were

encapsulated by the zinc source in aqueous solution, dried, and subjected to heat treatment. The heat treatment underwent by the amorphous SiO2 nanoparticles, with zinc source

mix-ture, showed the changing of phases, morphology, and size with increased temperature. ZnO phase appeared at the beginning of heat treatment and Zn2SiO4phase started to emerge at

800◦C onwards, as shown by XRD patterns. The average crystallite size increases from 37 nm at 600◦C to 68 nm at 1000◦C. The spherical morphology was observed at 600 and 700

C, but at temperatures higher than 800C, the dumbbell or necking-like structures formed.

Optical band gap analysis of Zn2SiO4 composite was determined to be within the range of

3.12±0.04 to 3.17±0.04 eV. The photoluminescence of treated samples showed emission peaks at 411 and 455 nm wavelengths from ZnOs blue band and at 528 nm wavelength from Zn2SiO40s green band. The diffusion of zinc ions into Zn2SiO4 composite with high surface

area will favour the diffusion at a much lower temperature compared to a conventional solid state method.

Keywords: Zinc silicate, oxide materials, photoluminescence, amorphous silica, nanopar-ticles

I. INTRODUCTION

In recent years, interest on the synthesis and

analysis of Zn2SiO4 composite has grown due

Corresponding author:engku ghapur@umt.edu.my

to its potential in display devices, detector

sys-tems, such as X-ray screens and scintillators, and

for luminous paint, or coating. Previous studies

have focused on improving synthetic methods,

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ma-terials. Traditionally, Zn2SiO4 is prepared using

a solid state reaction method by applying heat

treatment to a mixture of zinc oxide (ZnO) and

silica (SiO2) as the precursors [1]. The physical,

chemical, and optical properties of the resulting

Zn2SiO4depend on the preparation method and

starting materials. As for silicate-based

phos-phor, the selection of silica precursors can

cer-tainly influence the final properties of the

tar-geted materials.

Other from the laboratory grade SiO2, soda

lime silica produced from waste materials

[2]-[3], silicon alkoxide, or tetraethyl orthosilicate

(TEOS) [4], and commercialized mesoporous

SiO2 are among common SiO2 sources [5]. The

conventional melt quenching of soda lime

sil-ica, mixed with ZnO, produces a glass

precur-sor, which is then subjected to heat treatment

to form Zn2SiO4-based glass-ceramic.

The drawback of using soda lime silica is the

presence of other elements in the glass, which can

affect the properties of the final materials [6]. Sol

gel, hydrothermal and vapour method require a

specific materials or equipment in order to

pre-pare the samples. Requirements such as soluble

precursors for gel-like system [7], a Teflon-lined

autoclave to heat the mixtures for hydrothermal

method and a chamber or container for vapour

method is needed for produce the materials

[8]-[9].

The use of silica, whether solid or porous

particles as a precursor, has the possibility of

adopting other methods that favour simple

cess routes and lower cost. This technique

pro-vided a simple preparation route that did not

require special equipment during synthesis [10].

The use of solid silica mixed with a source of

zinc ions in an aqueous state can influence some

of the properties and changes in the phase

for-mation temperature. Porous or mesoporous

ma-terials have been used to synthesise mama-terials or

compounds that can form new functional phases

that will improve the properties of the initial

source material. In this study, the wet chemical

impregnation method, combined with sintering,

was used to synthesise Zn2SiO4composite. Solid

SiO2 nanoparticles were used as a template and

mixed with a solvent containing dissolved zinc

ions from zinc nitrate hexahydrate.

II. MATERIALS AND METHODS

The synthesis of SiO2 nanoparticles powder

are prepared according to the procedure

pre-sented in earlier report [11]. Zn2SiO4

compos-ite powders were produced by mixing

amor-phous silica nanoparticles that were produced

previously with zinc nitrate hexahydrate. First,

1.20 g of amorphous silica nanoparticles was

dis-persed in 200 mL of deionized (D.I.) water

us-ing an ultrasonic agitator for 30 minutes. Next,

the amorphous silica nanoparticle dispersion was

mixed with a solution of 11.89 g of zinc nitrate

hexahydrate (Sigma Aldrich), which was already

dissolved in 200 mL of deionized water. This

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stirrer. The ratio of the precursor concentration

of Zn:Si are 2:1, 1.75:1, 1.5:1 and 1.25:1. Then,

the mixed solution was poured into a petri dish

and dried in an oven for 24 hours at 120◦C. The dried powder was ground using a mortar grinder

and finally, heat treated at 600, 700, 800, 900,

and 1000 ◦C at 10 ◦C /min heating rate for 3 hours.

To study the structural characteristic of

syn-thesised Zn2SiO4 composite, the sample powder

was characterized by X-ray diffraction (XRD)

(PANalytical X0pert PRO PW 3040 MPD), field emission scanning electron microscopy (FESEM)

(FEI NOVA NanoSEM 230) and Fourier

Trans-form Infrared reflection (FTIR) spectrometer

(Perkin Elmer Spectrum 100) with universal

at-tenuated total reflectance (ATR) between the

wavenumber of 400 to 4000 cm−1. In addition, the optical property was characterized using

UV-Visible (Shimadzu UV-3600) spectrometer with

absorption signal was measured between 200 to

800 nm wavelengths for optical absorption and

band gap determination. The diffuse reflectance

measurements give the reflectance values of the

sample. To further analyse the optical

prop-erties of the sample, the Kubelka-Munk model

was used to determine the absorption coefficient

using the specific software that came with the

UV-Vis equipment. Perkin Elmer LS 55

Flu-orescence Spectrometer was used in the

photo-luminescence characterization analysis and the

wavelength of 350 nm was chosen to excite the

material.

III. RESULTS AND DISCUSSIONS

A. XRD analysis

Figures 1 shows the XRD spectra of heat

treated Zn2SiO4 composite were recorded in the

range 2Θ = 10◦ to 80◦form 600 to 1000◦C. The wurtzite structure of the ZnO nanocrystals was

observed at 600◦C. Nine major diffraction peaks were positioned at 2Θ values that corresponded

to the X-ray diffraction planes of ZnO wurtzite

crystal structure (JCPDS No. 89-0510). The

lattice constant calculated from the XRD

spec-trum gave the value of a = 3.249 ˚A and c =

5.206 ˚A, which were similar to ZnO hexagonal

wurtzite structure [12].

Diffraction peaks had broadened and shifted

towards the higher angle side with increased of

sintering temperature. When the size of ZnO

nanocrystals had increased, its growth might has

been interrupted by the surrounding SiO2

ma-trix and porous nature of the SiO2 nanopowder.

As a result, the diffraction peaks of ZnO were

shifted towards the higher angle side, with

in-creases in temperature that related to the strain

induced in the ZnO nanocrystals by the

sur-rounding silica network [13].

At 700◦C, the small peaks of Zn2SiO4 phase started to form and became obvious at 800 ◦C that corresponded to α-Zn2SiO4 crystal phase

(JCPDF No. 37-1485). The formation of

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dif-fusion species during thermal annealing due to

its lower activation energy of surface diffusion

[14]. The activation energy was gained rapidly

with the increase in temperature. The zinc ions

on the surface of the SiO2 nanoparticles had

diffused and formed Zn2SiO4 at lower

temper-atures. Furthermore, using SiO2 nanoparticles,

which contained silanols, amorphous in structure

and large surface area has led to the lowering of

the formation temperature [5]

Additionally, the impregnated ZnO on the

surface of silica nanoparticles has low surface

diffusion activation energy. The lower

activa-tion energy (158 kJ/mol) of the ZnO surface

dif-fusion compared to the bulk difdif-fusion (347-405

kJ/mol) will primarily diffuse the ZnO atoms at

the surface towards the silica matrix and

pro-mote the formation of Zn2SiO4 crystal phase

[15]. During this time, the splitting of zinc oxide

peak at 31.79◦and 34.44◦, which shifted to lower 2Θ was also observed. The rise in temperature

had immediately increased the diffusion. Hence,

Zn2SiO4 crystal phase formation had proceeded

with the increase in sintering temperature and

at 1000 ◦C, Zn2SiO4 became the main crystal phase and the intensity of ZnO peak had

de-creased. The single phase of Zn2SiO4 was likely

to have been formed with a small amount of ZnO

trace was still available at 1000 ◦C.

The full width at half maximum (FWHM)

values were used to calculate the average

crys-tallite size using the Scherrer0s equation [16]

Figure 1. XRD pattern of Zn2SiO4 composite,

heat treated between 600 to 1000 ◦C.

D =βcosΘkλ ...(1)

wherek = Scherrer constant depends on the

how the width is determined, the shape of the

crystal and the size distribution, λis the X-ray

wavelength (1.5418 ˚A), β is full width at half

maximum in radians (FWHM) and Θ is Bragg

angle in degree. As the heat treatment

tempera-ture had increased, the average calculated

crys-tallite size of ZnO, has also increased (Table 1).

At 600◦C, the average crystallie size was approx-imately 37 nm, and increased between the ranges

of 68 nm at 1000◦C of heat treatment. The av-erage crystallite size of Zn2SiO4 crystal phase of

the same samples had similar increments of

crys-tallite size values with increasing temperature.

The lattice constant of Zn2SiO4 phase

calcu-lated from the sintered sample was about a ∼

13.932 ˚A and c ∼ 9.257 ˚A, which was close to

the values obtained by Ghoul and Mir (2015),

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sil-ica precursor [16]. The increasing temperature

has made the Zn and Si ions to become moveable

at the contact surface due to the increased

en-ergy. These ions tend to diffuse and move into

the SiO2 nanoparticles, thus forming Zn2SiO4

phase.

Table 1. The average crystallite size of ZnO

and Zn2SiO4 at various sintering temperatures.

Temperature (◦C) Average crystallite size (nm) ZnO Zn2SiO4

600 37.30

-700 40.30

-800 42.12 43.51

900 54.29 45.40

1000 66.05 52.09

B. FTIR analysis

Figure 2 shows the FTIR spectra of Zn2SiO4

composites sintered at various temperatures.

The sample that was dried at 120 ◦C showed IR bands at 3600 cm−1 and 1600 cm−1, which can be attributed to the O-H stretching

vibra-tions and bending modes of adsorbed water. The

presence of organic molecules were represented

by the IR bands at 1300 cm−1 for C-H bond and 1320 cm−1 (weak shoulder peak) for NO−3 bond. This bond can be referred to nitrate

ions in nitrate and some organic contaminants

that were in the precursors and de-ionized

wa-ter. The mixture of Zn and Si ions was added

with hydrolysed zinc nitrate and SiO2

nanopar-ticle powder in deionized water from the

previ-ous preparation step. Yang et al. (2008)

re-ported that the electrostatic force could

influ-ence Zn(OH)SiO2 colloidal particles to be

ab-sorbed onto SiO2 nanoparticles [17].

At 600 ◦C, the entire organic and hydroxyl bonds have disappeared. Meanwhile, the IR

bands that existed for all heat treated samples

were observed at 1050 to 1100 cm−1, 790 to 880 cm−1, and 410 to 360 cm−1 assigned to the asymmetric stretching modes of Si-O-Si

vibra-tions, symmetric stretching of ZnO4, and ZnO

stretching band. The amorphous nature of

sil-ica dried at elevated temperatures prompted the

availability of silanol or hydroxyl groups. These

water-related molecules were present at the

sur-face, as well as within the pores inside the

par-ticle structure [18]. When the hydroxyl groups

at the surface of the silica particles became

con-densed, the dangling bonds on the silica surface

would attach to the zinc phase in ZnO. This

would allow the consolidation and diffusion to

occur between SiO2 and ZnO atoms. With

pro-gressive sintering temperature, the intensity of

the 1056 cm−1peak had gradually decreased and shifted to 1110 cm−1. This result suggested that Si atoms were replaced by Zn ions to form

Si-O-Zn bonds [19].

New vibrational bands were observed at 570

to 600 cm−1, linked to the symmetrical vibra-tions of the ZnO4 group [20]. The group of

vi-brational bands observed between 790 and 880

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temper-ature, which can be attributed to SiO4

tetra-hedron vibration. The presence of vibrational

bands of SiO4 and ZnO4 groups suggests the

for-mation of the Zn2SiO4 phase [21]. The bands for

asymmetrical stretching of Si-O-Si and ZnO had

decreased, which indicated that the Si-O-Si and

ZnO bonds were broken during the conversion to

Zn2SiO4.

Figure 2. FTIR spectral pattern of heat treated

Zn2SiO4 composite at a temperature ranging

from 600 to 1000 ◦C.

C. FESEM analysis

Figure 3 shows FESEM images of samples

ex-hibited spherical morphology at 600 and 700◦C, but at temperatures of higher than 800 ◦C, the dumbbell or necking-like structures were formed.

The overall morphology of Zn2SiO4 composite,

upon annealing at 600◦C, showed the spherical form with an average diameter of 69.68 ± 3.20

nm. Some parts of the composite displayed

in-dividual particles, while other parts showed

ag-glomeration. Burned off residual organic groups,

such as nitrates, could lead to the encapsulation

of ZnO on the surface of SiO2 nanoparticles and

the particle size cannot be precisely estimated

from annealing at 700 ◦C [14]. For the sample that was heat treated at 800◦C and higher, the particles appeared to be diffusing into one

an-other. At 1000 ◦C, the melting-like morphology was observed [22]. The formation of bulky

struc-tures at this temperature could possibly be due

to the tendency of the spherical particles to

min-imize their surface energy by diffusing with one

another at elevated temperatures.

D. UV-Vis spectroscopy analysis

Figures 4 shows the absorption spectra of the

UV-Visible for heat treated Zn2SiO4 composite

recorded in the spectral region of 200 to 800

nm. The maximum absorption edge of the heat

treated samples was at approximately 380 nm.

This sharp absorption edge correlate to the

char-acteristics of the crystalline phase of ZnO. For

sample heat treated at 600◦C, when the ratio of zinc was decreased, the absorption intensity

be-came lower. Gun0Koet al. (2013) suggested that the intensity of the band was highly depended on

the content of zinc oxide [23].

E. Optical band gap analysis

The electronic transitions of crystalline or

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analysing the resulting optical absorption

coeffi-cient (α) near the absorption edge. The

experi-mental optical band gap can be obtained by

us-ing equation (αhν)1/n = A(hν - Eg), where A is

a constant, the photon energy is denoted by hν,

and Eg is the optical energy band [3],[24]. The

determination of optical band gap value from

the best linear fit by extrapolating the linear

parts of ((αhν)1/n vs. hν curves is done by

us-ing n = 1/2 which is value for direct allowed

transition. The x-axis interception from

extrap-olation of linear region of individual plot lines

gave optical band gap value as shows in Figure

5. The band gap for ZnO of heat treated

sam-ple in the range of 3.12±0.04 to 3.17±0.04 eV,

compared to the bulk band gap which is 3.37

eV. Sideket al. (2009) emphasised that optical

band gap decreased with the additional of

non-bridging oxygen (NBO) in the TeO-ZnO system

[25]. The symmetrical vibration of the ZnO4and

SiO4 group observed in FTIR spectra with

in-creasing of temperature causes the increment of

NBO. The shift in UV absorption band is

notice-ably from NBO that bound an excited electron

lesser than bonding oxygen (BO). This factor

contributes to the variation in optical band gap

of the heat treated samples. The higher

sinter-ing temperature also had increased the particle

size and became denser structure with less pores,

due to diffusing of articles with one to another

as shown in FESEM micrographs that could lead

to higher absorbance of photons [26].

Figure 3. FESEM micrographs of Zn2SiO4

composite heat treated at: (a) 600◦C; (b) 700

C; (c) 800C; (d) 900C; and (e) 1000C.

F. Photoluminescence analysis

Figures 5 shows the emission spectra of

Zn2SiO4 composites with the increase of heat

treatment. Several peaks were observed at the

particular wavelength, with differences in

inten-sities. Based on the XRD spectra, the higher

heat treatment temperature would increase the

crystallinity of the ZnO and Zn2SiO4phases. By

applying the Gaussian fitting, four peaks can be

resolved from the spectra, observed at 411, 455,

480, and 528 nm wavelengths. The near-violet

emission at 411 nm of the ZnO-Zn2SiO4

compos-ite was reported by Vaishnavi et al. (2008)

sug-gested that the interface traps in the depletion

region were due to the oxygen vacancies at the

ZnO-SiO2 interface formation in the [27]. The

blue emission at 455 and 480 nm wavelengths

attributed to oxygen vacancies inside ZnO

crys-tallites and interstitial oxygen. The

photolumi-nescence intensity was reduced when the

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Figure 4. (a) UV-Vis absorption spectra for Zn2SiO4 composite, sintered from 600 to 1000 ◦C. (b) Tauc plot of (αhν)2 vs. energy for Zn2SiO4 composite, with Zn:Si ratio of 1.25:1, sintered at

600 to 1000◦C using SiO90m.

which was caused by the reduction in oxygen

vacancies. At 1000◦C, the emission peak at 528 nm had increased for all treated samples. He

et al. (2003) suggested that although the

for-mation of Zn2SiO4 can be detected after

under-going high heat treatment temperature (higher

than 900 ◦C), the Zn2SiO4 phase did not con-tribute to the emission of the green band [28].

The green band emission was indicated as the

transition of the photoexcited electron from the

conduction band edge to an oxygen vacancy in

ZnO. In this study, it is apparent that no

activa-tor was used on the host material. The emission

at 528 nm can be related to the changing of

en-ergy schematic of ZnO and the defects generated

by the neighbouring crystal field of Zn2SiO4

dur-ing heat treatment.

IV. SUMMARY

In this study, Zn2SiO4 composites were

suc-cessfully prepared by sintering the mixture of

SiO2 nanoparticles with aqueous zinc nitrate.

The formation of Zn2SiO4 composite was

ob-served when the temperature was increased to

higher than 800 ◦C, which was lower than con-ventional solid state sintering temperature. The

average crystallite size increases from 37 nm at

600◦C to 68 nm at 1000 ◦C. The impregnation of zinc ions with amorphous SiO2 nanoparticles

had significantly reduced the formation

temper-ature of Zn2SiO4 phase. The Zn-O and

Si-O-Zn vibration bands in the FTIR transmittance

spectra shows the characteristics of Zn2SiO4

phase formation. Based on the morphological

analyses from FESEM, as the temperature

in-creases, the spherical particles exhibited

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mea-Figure 5. Photoluminescence spectra of: (a) Zn2SiO4 composite, sintered at various

temperatures, and Gaussian fitting for samples sintered at: (b) 600 ◦C: (c) 700◦C: (d) 800◦C: (e) 900◦C; and (f) 1000 ◦C using SiO90m excited at 355 nm wavelength.

surements showed the values of the calculated

optical band gap of ZnO to be in the range

of 3.12±0.04 to 3.17±0.04 eV. The

photolumi-nescence spectra showed the emission of near

violet, blue, and green bands. The intensity

of band emission was mostly influenced by the

heat treatment temperature. The synthesised

Zn2SiO4 composite had shown as a promising

candidate for phosphor materials. Its optical

properties appeared suitable to be further

de-veloped for applications in photonic and

opto-electronic materials.

V. ACKNOWLEDGMENT

The authors gratefully acknowledged the

financial support for this study from the

Malaysian Ministry of Higher Education

(MOHE), Universiti Malaysia Terengganu and

Universiti Putra Malaysia.

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Figure

Figure 1. XRD pattern of Zn2SiO4 composite,
Table 1. The average crystallite size of ZnO
Figure 3. FESEM micrographs of Zn2SiO4
Figure 4. (a) UV-Vis absorption spectra for Zn2(b) Tauc plot of (SiO4 composite, sintered from 600 to 1000 ◦C.αhν)2 vs
+2

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